1. Field of the Invention
The present invention relates to an image capturing apparatus such as a digital camera or video camera.
2. Description of the Related Art
Japanese Patent Laid-Open No. 2008-52009 discloses an apparatus which performs focus detection using the pupil division scheme using a two-dimensional image pickup device having microlenses formed on the respective pixels as one of the schemes of detecting the focus state of an imaging lens. In this apparatus, the photo-electric conversion unit of each pixel constituting an image pickup device is divided into a plurality of units, and the divided photo-electric conversion units receive light beams passing through different areas of the pupil of the imaging lens through the microlens. This apparatus performs correlation calculation for calculating a phase difference as a shift amount for a pair of output signals from the photo-electric conversion units which have received light beams passing through different areas of the pupil of the imaging lens, and can calculate a defocus amount from the phase difference.
When performing the above correlation calculation, the apparatus preferably can calculate phase differences in units finer than the pixel pitch of an output signal, or in other words, subpixels, with high accuracy, in order to accurately calculate phase differences. When performing correlation calculation using a pair of output signals, the scheme disclosed in Japanese Patent Laid-Open No. 2008-52009 also performs interpolation calculation for the minimal value of a correlation amount by using a three-point interpolation technique to calculate a phase difference.
Depending on the phase difference between a pair of output signals, however, the technique disclosed in Japanese Patent Laid-Open No. 2008-52009 sometimes lets the interpolation calculation result on the minimal value be greatly influenced by an error. It is possible to reduce the error contained in the phase difference calculated from the first and second signals by using the phase difference calculated by using the second signal and the third signal different in sampling position from the first signal on an object image by half a pixel. However, the focus detection apparatus described in Japanese Patent Laid-Open No. 2008-52009 has not disclosed any method of implementing this technique.